1. Technical Field
This invention relates generally to monobloc pistons for diesel engine applications in which the piston head is formed with an internal oil cooling gallery and includes a piston skirt fixed to the pin bosses and ring belt of the head, and more particularly to the management of lubricating oil scraped from the cylinder walls of the engine during operation of the piston.
2. Related Art
Diesel engines often employ so-called monobloc pistons in which the piston head and skirt are unitized, such that the skirt is immovable relative to the head. The piston head is formed with an internal cooling gallery in which cooling oil is circulated to cool the head during operation.
The piston head includes an outer ring belt in which a number of circumferentially continuous ring grooves are formed. The ring grooves accommodate corresponding piston rings that ride against the wall of the piston cylinder during operation, in known manner. Typically, the lowest of the ring grooves accommodates an oil scrapper ring which serves to scrape oil from the walls of the cylinder in order to limit the amount of oil that passes by the remaining compression rings to minimize xe2x80x9cblow-byxe2x80x9d and thus hydrocarbon emissions of the engine. Despite best efforts, a certain amount of oil does get by the compression rings. During the downstroke, the oil that is scraped from the walls builds up ahead of the rings and gets trapped between the walls of the piston and the cylinder. Consequently, it is believed that a hydrodynamic film of captured oil builds at the inner face between the oil scrapper ring and, despite high contact pressure, it is caused to escape past the rings.
It is an object of the present invention to improve upon such monobloc pistons by providing better management of the oil scrapped from the cylinder walls to reduce blow-by and thus hydrocarbon emissions of such pistons.
A monobloc piston constructed according to a presently preferred embodiment of the invention includes a piston head having a combustion bowl formed in an upper surface of the head and an outer ring belt formed with a plurality of ring grooves in an outer surface of the ring belt. A cooling gallery is formed in the piston head having an outer wall provided at least in part by the ring belt. A pair of pin bosses extend from the piston head opposite the combustion bowl and are formed with axially aligned pin bores. The pin bosses have outer side faces that are spaced laterally inwardly of the outer surface of the ring belt. A piston skirt is fixed at its upper end to the ring belt and is formed as one piece with the pin bosses. According to the invention, the piston head is formed with at least one oil drainage groove below the lowest of the ring grooves which is free of piston rings and open to the outer surfaces of the pin bosses. The invention also contemplates a method of forming a monobloc piston having such features.
The invention has the advantage of providing a monobloc piston with improved oil scrapping management, and more particularly improved drainage of the oil scraped from the walls of the piston to minimize escapement past the rings. The improved oil drainage reduces the buildup and back pressure of oil ahead of the oil scraping ring during the downstroke of the piston, thus allowing more of the oil to be scraped from the walls and less of the oil to escape past the rings, for a recognized reduction in the emission of hydro-carbons from such diesel engines.